1. Field of the Invention
This invention relates to thermally insulating building materials of an indeterminate ccomposition, which are prepared by low-temperature treatment of siliceous rock having a high content of amorphous silicon dioxide, usually higher than 70%, preferably higher than 80% by weight.
Such materials can be in particulate final state suitable for use as a loose thermal insulation, as aggregate in making mainly light-weight concrete, and as slabs or blocks which in most cases may be immediately used preferably as cladding for buildings and various structures.
2. Description of the Prior Art
The above-defined materials are produced on a mass scale and are under various conflicting requirements that are becoming increasingly more rigid.
Actually, it is very desirable that such materials exhibit as low bulk density (for loose particles) or density (for slabs and blocks) as possible and the lowest possible heat conductivity while offering the greatest possible strength and weather resistance, particularly under conditions characterised by freeze-defrost cycles.
It is further desirable that such materials be combinable with inert ingredients that will modify their mechanical and/or thermal properties, and be capable of production as final loose particles of arbitrary sizes and shapes or as blocks and slabs from readily accessible raw materials in a production process of a minimum power consumption, the product quality variations being kept within the narrowest range possible.
The above requirements can be now satisfied either separately or in some combinations only.
The siliceous raw materials, which are really not uncommon in the manufacture of thermally insulating building materials, are readily accessible. Among such raw materials belong siliceous minerals, such as diatomite, tripoli, gaize, spongolite, radiolarite, and their artificially prepared analogs. Of these latter materials, sodium silicate, e.g. soluble glass, is the best known and commercially available.
Thermally insulating building materials having relatively low bulk density (less than 1000 kg/m.sup.3) and low heat conductivity are known to be obtained by preparing compositions capable of being heat expanded, forming intermediate products (preferably by agglomeration and particularly by pelletizing or jet granulation), and by expanding the intermediate products at high temperatures, usually above 800.degree. C.
Building materials of this kind may be examplified by a porous aggregate used in making mainly light-weight concrete and produced from siliceous rock containing from 30% to 98% by weight of silicon dioxide, no more than 20% by weight of aluminum oxide, no more than 25% by weight of calcium oxide and some other constituents. To produce such aggregate, an appropriate natural raw material is crushed, then roasted (usually in rotary kilns) at temperatures from 1080.degree. C. to 1380.degree. C., and the resulting product is cooled (V. N. Ivanenko, `Building and civil engineering materials and shapes from siliceous rock`, published 1978, by BUDIVELNYK Publishers (Kiev), see pages 49 to 58 [in Russian]).
Cracks visible to the unaided eye can always be found in such materials, this resulting in significant water absorption and a correspondingly low frost resistance. Also, high-temperature roasting involves an economically unacceptable specific power consumption.
More preferable are thermally insulating building materials produced from a process alkalized and wetted siliceous raw material by roasting at lower temperatures.
The term "alkalized", as used herein, refers to such siliceous raw materials in which a hydroxide of an alkali metal (preferably caustic soda) is inherently present or added, and the term "wetted" means that water is used during preparation of a raw mixture at least as an ingredient indispensable for preparing intermediate products.
Thermally insulating building materials produced from such raw materials may be examplified by porous aggregate for light-weight concrete (artificial "gravel" or "sand") and thermally insulating boards ("foam glass") (See V. N. Ivanenko, pages 102 to 103 and pages 98 to 99, respectively, referred to above).
Production of such materials consists in preparation of a raw mixture of 100 parts by weight of powdered siliceous rock with a particle size of up to 0.14 mm, from 8 to 22 parts by weight of a hydroxide of an alkali metal, i.e. caustic soda or caustic potash, and from 18 to 38 parts by weight of water, obtaining an intermediate product and roasting it at a temperature of 1180.degree. C. to 1200.degree. C. to attain expansion.
The materials produced by the above-outlined process, compared with the earlier-mentioned porous aggregate, do have cracks resulting in high water absorption and a low frost resistance, though the process is moderately less power intensive.
Known in the art are materials based on the above-mentioned soluble glass, whose characteristics in terms of integrity, water absorption and frost resistance are better, since the soluble glass inherently contains from 6 to 20% of a hydroxide of an alkali metal. Soluble glass must be powdered and water must be added in the ratio of 9:1 by weight. Then the wetted powder is formed into an intermediate product which is steamed in a gaseous atmosphere containing more than 50% of superheated steam at 100.degree. C. to 200.degree. C. and above 0.1 MPa gage pressure. The steamed intermediate product is further heat treated (dried and/or calcined) at a temperature above 100.degree. C., preferably above 800.degree. C., to attain expansion (U.S. Pat. No. 3,498,802).
Hydrous sodium or potassium silicates present in the starting material significantly promote the forming of the intermediate product and provide for a noticeable reduction in specific power consumed in the heat treatment step.
Nontheless, it is doubtful that porous materials of low (less than 1000 kg/m.sup.3) bulk density can be obtained from such raw materials at temperatures below 800.degree. C.
The intermediate products at temperatures so high are expanded mainly due to polymorphic transformations in silica to result in a final product of loosened up structure and low mechanical strength, for one thing, and of above-mentioned increased water absorption and reduced frost resistance due to disintegration of surface layers, for the other. In some cases the strength of the product is too low for shipment to the site where it is to be used.
An appreciable increase in the strength of thermally insulating building materials, a reduction in water adsorption, and a reduction in specific power consumed in the production process have been disclosed in Ukrainian Patent No. 3802.
A thermally insulating building material of the above type, based on an alkalized and wetted siliceous raw material (containing in particular from 1 to 30 parts by weight a hydroxide of an alkali metal and from 30 to 125 parts by weight water per 100 parts by weight of the siliceous material) is obtained by comminuting the solid ingredients and mixing all the ingredients, steaming the mixture (specifically in the atmosphere of a saturated steam at a temperature of 80.degree. C. to 100.degree. C. for 20 to 60 min), to produce an intermediate product (usually by pelletizing) and heat expanding (e.g. at a temperature of 150.degree. C. to 660.degree. C.) the intermediate product. This is the material that bears closely on the invention.
Compared with other prior art materials, this one can be produced by a process that is the least power intensive, and with a comparatively low (from 50 to 950 kg/m.sup.3) and readily controllable bulk density, it is noted for a wholly satisfactory porosity such that water adsorption is no more than 32.5% at worst, on the one hand, and sufficient mechanical strength, on the other.
These advantageous features are due to the steaming of the mixture which turns into a gelled viscous sticky low-melting mass based on hydrosilicates of alkali metals. This mass is substantially impermeable to low pressure gases and steam which are common to the steaming step under indicated temperature conditions, but when expanded it is slightly permeable to gases and steam.
The low-melt mass admits of low expanding temperatures and a corresponding low specific power consumption in the production of a final product, while the relative gas-impermeability of the mass provides for the indicated satisfactory porosity and sufficient strength.
However, the final thermally insulating building material produced according to Ukrainian Patent No. 3802 does not possess invariable qualities. The strength of pellets varies in the range of 0.02 to 12.5 MPa, and water adsorption in the range of 4% to 32.5%. What is more, in practice, on attempted control of pellet size in loose material it has appeared that the smaller the medium pellet size, the greater the number of rejected pellets due to aggregation or conglomeration of the pellets in the pelletizing and heat expanding steps; and on an attempted manufacture of building products such as blocks and slabs, negligible cracks and a total absence of open pores in the surface layers of intermediate products have proved to be obstacles to gases to be driven off during the expansion step, and the larger the building product, the greater obstacles. Building products with some (varied with composition) excessive dimensions have all been rejected.
This undesirable outcome is due to a combination of high viscosity and stickiness of the steamed mixture with its low gas permeability.
Attempts at reducing such outcome, by adding from 1 to 150 parts by weight of an inert mineral aggregate filler incapable of combining under said conditions of steaming into hydrosilicates of alkali metals, have not been successful in noticeably doing away with quality variability in the final product.